Hot melt adhesives are applied to a substrate while in a molten state and cooled to harden the adhesive layer. Such adhesives are widely used for various commercial and industrial applications such as product assembly and packaging, and have been widely used in the nonwoven industry to make nonwoven articles. In these applications, adhesive is applied to at least one substrate for binding the substrate to a second similar or different substrate.
In a hot melt adhesive, base polymer provides cohesive strength and elasticity. The use of high molecular weight polymer or high polymer content usually promotes cohesive strength and adhesion; however, it also results in significant melt viscosity increase. A hot melt adhesive with high viscosity may require very high processing temperature, at which polymers are susceptible to degradation, charring, gelling and loss of adhesion. Moreover, high processing temperature increases energy cost, creates a safety hazard and deforms/discolors polymeric substrate film.
Conventional low application temperature hot melt adhesives, e.g. styrene-butadiene-styrene, styrene-isoprene-styrene and metallocene polyolefin based adhesives, are known, however, amorphous poly-α-olefin based adhesives are not processed at low temperature unless large quantities of low molecular weight diluents are added to lower the adhesive viscosity. Adding high levels of diluents, such as tackifiers and waxes, decreases mechanical strength of the adhesive, and more importantly, causes less resistance to flow at body temperature. Also, over time, the low molecular weight diluents in the adhesive tend to migrate and separate out from the adhesive further affecting the strength and appearance of the adhesive.
There is a need for a hot melt adhesive that can be applied at low temperature, i.e., below about 150° C., which has lower amount of diluents, without sacrificing the mechanical strength and appearance of the adhesive. Such attributes would make the adhesives particularly well-suited for use in the manufacture of absorbent disposable articles. The current invention addresses this need.